1. Field of the Invention
The present invention generally relates to the measurement of pressure, strain, temperature, or displacements in a high temperature environment and, more particularly, to an optical interferometer capable of measuring sub-nano meter displacements at temperatures over 2000.degree. C. The invention is useful in detecting an optical path length change which may result from a variety of causes in a high temperature environment.
2. Description of the Prior Art
Measurement devices which employ optical fiber elements were first described in the late 1970s. Typically, they use optical fibers either as a light transmission element or as an element which is modified by external phenomena. The most sensitive optical fiber measurement devices developed thus far are based on optical interferometry. Optical fiber interferometers which use single mode silica optical fiber have been applied to a wide range of measurement problems during the 1980s.
The measurement of environmental parameters such as displacement, temperature, strain and pressure at high temperatures is increasingly difficult because the materials used to fabricate the sensing elements deform or melt at sufficiently high temperatures. The softening temperature of silica optical fibers is typically between 900.degree. C. and 1000.degree. C. At temperatures equal to or above this, silica optical fibers are not useful, even if they are coated with materials which melt at higher temperatures.
One approach to fiber-based measurements at higher temperatures is to use fibers made of sapphire. Currently, sapphire fibers are only made in the form of rods of diameters of 50 microns or larger, and the rods lack a cladding or a coating similar to those which are fabricated as part of conventional silica fibers. The large diameter of the sapphire rod materials does not allow single mode optical propagation and, thus, does not allow the implementation of optical fiber interferometers of the silica single mode fiber type described above.
Optical fiber measurement systems have been implemented using sapphire rods as a transmission devices to transport optical signals in a region of high temperature. Since the melting temperature of sapphire is above 2000.degree. C., operation at temperatures much higher than those obtainable with silica fibers is possible. Applications in the past have only included using such sapphire rods as simple transmission devices, and not as elements in optical interferometer configurations. Those applications have specifically included the use of sapphire rod materials as a "window" to observe the temperature in a hot region, or as an infrared transmission element to observe the infrared absorption characteristics of the surrounding environment.
U.S. Pat. No. 4,859,079 to Wickersheim et al. and U.S. Pat. No. 4,883,354 to Sun et at., both assigned to Luxtron, Inc., use the principle of blackbody radiometry where the radiation from a body of known (or unknown) emmissivities is correlated to its specific wavelength properties. By using ratio-pyrometry, that is, using information at multiple wavelengths and taking ratios to remove extraneous effects, temperature information is obtained. The use of multiple wavelengths does not involve any interferometric technique. Japanese laid open application 61178624 discloses a high temperature measurement device which uses a thin film made of platinum or iridium at the tip of a sapphire fiber. As the temperature of the surface changes, the radiation energy from the film is measured and information from the spectral brightness is used to acquire temperature information.
U.S. Pat. No. 4,918,492 to Ferdinand et al. discloses a Michaelson interferometer in which optical fibers are provided between a light source and an optical coupler, from the coupler to a collimator, and from the coupler to a photodetector. There is no optical fiber exposed to high temperature. Each of the optical fibers used by Ferdinand et al. are single mode fibers, and the use of a multimode fiber in the Ferdinand et al. interferometer would render the device inoperable. None of the fibers in the Ferdinand et al. interferometer are themselves part of the interferometer per se; rather, they provide only light conducting paths.
U.S. Pat. No. 4,750,139 to Dils discloses a blackbody radiation sensing optical fiber thermometer system that employs a sapphire rod terminated in a black body tip composed of iridium sputtered onto the end of the rod. The sapphire rod of Dils in not used in an interferometer system.
U.S. Pat. No. 4,627,731 to Waters et al. discloses a heterodyne optical interferometer which is particularly adapted to measuring the vibration of a workpiece. Two light beams having different optical paths are chosen so that the two light beams are incoherent. These two light beams are modulated by respective Bragg cell acousto-optic modulators, and the modulated light beams are combined in a coupler and provided to an optical fiber. Since the two modulated light beams are incoherent, no interference occurs between the beams propagating in the optical fiber. A portion (i.e., the reference beam) of the two modulated light beams is internally reflected from an end face of the optical fiber, and the remaining power of the combined beam (i.e., the measurement beam) exits the end of optical fiber, is collimated by a first lens and focused by a second lens on a workpiece surface. The reflected light from the work surface traverses the two lenses and reenters the optical fiber. The length of the optical path traversed by the measurement beam is selected so that the first beam portion thereof has an overall optical path length that is approximately equal to the overall second beam optical path length to within the coherence length of the laser diode. As a result, the second beam portion of the reference beam and the first beam portion of the measurement beam will interfere because they are once again coherent. This interference yields a frequency modulated (FM) beam with a carrier frequency equal to the frequency of the optical modulators and a modulation signal proportional to the vibration of the workpiece surface. Thus, Waters et al. make vibration measurements at low temperature with FM heterodyne apparatus. The FM heterodyne interferometric technique employed by Waters et al. cannot measure quasi-static parameters such as pressure, strain, temperature, or displacements in a high temperature environment.
U.S. Pat. No. 4,679,934 to Ganguly et al. uses a black body radiating member on the end of bundle of sapphire fiber elements. Optical signals from the black body radiating member are conducted along the sapphire fiber bundle to a low temperature fiber bundle. The light output from the low temperature fiber bundle is collimated by a first lens, passed through a filter and focused by a second lens before being split by a beam splitter into two beams. These beams are passed through respective optical density attenuators before being detected by photodiodes. The beam splitter passes 90% of the light beam to one of the photodiodes and only 10% to the other photodiode, and the purpose of this arrangement is to increase the dynamic range of the measurement. The bundle of sapphire fiber elements used by Ganguly et al. is not a sapphire fiber, and it is used only to conduct light from a black body radiator and is not part of an interferometric system.